Nitrogen vapor engine

ABSTRACT

An open-cycle thermodynamic engine using a working fluid which is a gas at ambient conditions and which is liquefiable at temperatures below ambient. The engine operates by compressing the working fluid, such as liquid nitrogen, with a pump, convectively heating the nitrogen in a heat exchanger, and isentropically expanding the nitrogen within an expandable chamber to produce shaft work. The cycle is repeated both without compression and with compression of the working fluid. To salvage the remaining energy in the nitrogen or other fluid, an exhaust engine is used. In one embodiment the exhaust engine uses an expandable chamber and piston driven by a plurality of long, small diameter heat responsive wires coupled under tension between the chamber wall and piston. When gases of different temperatures are directed across the chamber, the wires expand and contract thereby reciprocally moving the piston. In another embodiment of the heat engine a manifold is connected to numerous closed tubes in which nitrogen is heated by natural convention. When the pressures within the tubes reach a maximum, the tubes are selectively exhausted against a double acting piston to produce work.

llnite States Patent [191 Manning et al.

[ 1 Jan. 22, 1974 NITROGEN VAPOR ENGINE [76] Inventors: Lindley Manning,4585 Clearview Dr.; Richard N. Schneider, 1950 Castle Way, both of Reno,Nev. 89502 [22] Filed: Sept. 23, 1971 [21] Appl. N0.: 182,994

Primary -ExaminerMartin P. Schwadron Assistant Examiner-Allen M.Ostrager Attorney, Agent, or Firm-Stephen S. Townsend et al.

SUPPLY TANK I l 'ST STAISE l l EXCHANGER: Q

WORK

WORK I ST STAGEl l l l OUTPUT EXPANDER OUTPUT EXPANDER IB [57] ABSTRACTAn open-cycle thermodynamic engine using a working fluid which is a gasat ambient conditions and which is liquefiable at temperatures belowambient. The engine operates by compressing the working fluid, such asliquid nitrogen, with a pump, convectively heating the nitrogen in aheat exchanger, and isentropically expanding the nitrogen within anexpandable chamber to produce shaft work. The cycle is repeated bothwithout compression and with compression of the working fluid. Tosalvage the remaining energy in the nitrogen or other fluid, an exhaustengine is used. In one embodiment the exhaust engine uses an expandablechamber and piston driven by a plurality of long, small diameter heatresponsive wires coupled under tension between the chamber wall andpiston. When gases of different temperatures are directed across thechamber, the wires expand and contract thereby reciprocally moving thepiston. In another embodiment of the heat engine a manifold is connectedto numerous closed tubes in which nitrogen is heated by naturalconvention. When the pressures within the tubes reach a maximum, thetubes are selectively exhausted against a double acting piston toproduce work.

4 Claims, 8 Drawing Figures AIR AIR i 2 ND STAGE5 5e RD STAGE:

HEAT -20 HEAT 2 EXCHANGER I EXCHANGER" r z r l 1 WORK 2ND STAGE WORK 3RD STAGE A 3VRD 24 g STAGE iRECOMPRESSlON I J PUMP H AIR 1 PATENIED JAN2 2 I574 SHEET 2 BF 5 80 MAIN COMPRESSOR PATENTEU JANE? 3.786.631

- sum w or 5 EXHAUST VALVE auc. 48o? NITROGEN VAPOR ENGINE FIELD OF THEINVENTION This invention relates to thermodynamic open cycle engines andmore particularly to engines having expandable chambers.

SUMMARY OF THE INVENTION One of the most serious contemporary problemsis the health hazard originating from the pollution of the atmosphere.The primary source of this air pollution is the unburned combustionproducts and exhaust gases released by internal combustion engines. Thetoxic combustion products released by these engines remain suspended inthe atmosphere for long periods of time and are ingested into the bodiesof all animals.

There has been a conspicuous failure to develop a suitable, practicalalternative to the internal combustion, gasoline engine which generatesmost of the air pollution. Currently, serious attempts are being made todevelop a steam engine for automotive use. In addition, there areprojects experimenting with batteries and electrical motors. As of yet,though, none of these projects has produced a practical, inexpensivealternative.

This invention contemplates an engine which operates on liquefied, inertgas. This engine does not operate by combustion nor does it releasetoxic combustion products into the atmosphere. Consequently, the exhaustdischarged by this engine would not be harmful to human beings or otheranimals.

This invention further contemplates an engine that will satisfactorilymeet the ever-tightening air pollution control laws regulating thedischarge of exhaust from automobiles. Currently, these air pollutioncontrol laws are becoming stricter each year and placing more burdensupon the designers of gasoline engines. These stricter requirements havealso resulted in substantial increases in the retail price ofautomobiles.

When comparing between engines, an automobile equipped with a nitrogenvapor engine is substantially lighter than a comparable automobilehaving a gasoline engine. This invention contemplates an engine whichdoes not require a cooling system with its associated radiator, fan,water pump, and engine coolant. Further, this invention contemplates asubstantially smaller engine having much smaller pistons than acomparable gasoline engine. In addition, this engine does not require anelaborate drive line having a clutch, a transmission, and a drive shaftall with their frictional losses. Mgs t likely the power requirement ofthis engine less than a comparable internal combustion en gine having aconventional drive line. Finally, this engine can positively eliminatethe raucous noise which accompanies unmuffled internal combustionengines.

An engine constructed according to this invention also provides asuitable source of power that can be used in environments heretoforeprohibited to gasoline engines. Vehicles constructed according to thisinvention can be used in ammunition and explosive storage andmanufacturing areas. In the past, gasoline engines have been excludedfrom these areas because of the hazard of explosions caused by the heatand the electrical sparks. An engine according to this invention canalso he used within an underground mine shaft where explosivc gases arepresent, the danger of the asphyxiation of nearby workers exists or thesupply of oxygen is limited.

By eliminating the need for gasoline to power automobiles, this enginecan conserve the world's petroleum reserves. Crude oil can be divertedfrom the production of gasoline to the generation of heat and themanufacture of chemicals. In addition, by eliminating the storage ofgasoline in automobiles, this engine substantially reduces the hazard offire and explosion present during automobile collisions.

These and other objects and advantages are met in accordance with thepresent invention by providing an open-cycle heat engine operable withliquefied nitrogen gas. The liquid nitrogen is obtained fromconventional sources and stored at atomospheric pressure andapproximately 77K in an insulated storage tank on the vehicle. Theengine cycle operates by first pumping the liquid nitrogen isothermallyfrom a pressure of one atmosphere to approximately 200 atmospheres. Nextthe nitrogen is passed through a convective heat exchanger therebyheating up the nitrogen isobarically from approximately 80K toapproximately 260K. From the heat exchanger the nitrogen is thenisentropically expanded within an expandable chamber, the piston ofwhich produces shaft work. The nitrogen is further isobarically reheatedin a second stage convective heat exchanger from approximately ll( toapproximately 260K at atmospheres of pressure. The nitrogen is againisentropically expanded in a second stage expandable chamber to produceshaft work. Following the second stage expander, the nitrogen isisentropically recompressed by a pump from approximately 12 atmospheresto approximately 20 atmospheres. The nitrogen is further isobaricallyreheated and isentropically expanded in subsequent stages producingadditional shaft work. The number of stages in any embodiment of thisinvention depends primarily on the required output power and the desiredefficiency.

The nitrogen vapor leaving the last stage of isentropic expansion flowsinto an exhaust engine. The exhaust engine salvages the last practicallyobtainable energy present in the nitrogen vapor. One embodiment of theexhaust engine according to this invention is a long, rectangular boxhaving a movable lateral wall or piston and numerous long, narrow gage,highly stressed wires that are strung between the movable lateral walland a stationary opposing wall. Cold nitrogen vapor from the exhaust andwarm ambient air are alternatively directed into the box. The coldnitrogen vapor causes the wires within the box to contract sharply whilethe warm air causes the wires to expand. The cyclic contraction andexpansion of the wires reciprocally drives the movable lateral wall thatis also mechanically linked to a crank shaft.

A further embodiment of the exhaust engine according to this inventionutilizes a double acting piston that is cyclically driven by nitrogenvapor. Cold nitrogen vapor at low pressure from the main engine flowsinto a bank of heat exchanger tubes. Within the tubes the nitrogen isheated by the ambient air thus raising its pressure. The now warmer andhigher pressure nitrogen is alternately directed against the opposingfaces of a double acting piston causing the piston to move reciprocally.Connected to the piston is a conventional connecting rod and crank shaftto provide the work output.

In both embodiments of the exhaust engine the last useful energy isremoved from the exhaust of the main engine and the nitrogen is ventedinto the atmosphere. The output of the exhaust engine drives the maincompressor and recompression pump. Thus, the exhaust heat engine takesotherwise unobtainable energy still within the nitrogen and increasesthe work output and efficiency of the main engine.

Almost any gas which is both a liquid at low temperatures andatmospheric pressure and a vapor at high pressure and atmospherictemperature can drive this engine. Nitrogen is primarily used because ofits vast supply in the atmosphere and because it is nearlyincombustible. Moreover, nitrogen has better physical properties thanmost other inert gases. At standard pressure, nitrogen is liquid at 320For 77K which gives liquid nitrogen a greater thermal potential withrespect to the atmosphere than either carbon dioxide or oxygen.

The engine according to this invention operates on the thermal potentialbetween the liquefied gas and the atmosphere. During the liqueficationprocess energy was removed from the gas and stored in the atmosphere.The atmosphere was thereby heated slightly. During the operating cycleof the engine, the liquefied gas is reheated by the atmosphere. Thus,the liquefied gas is acting like a small heat sink for the atmosphere.Shaft work is obtained from the redistribution of energy from theatmosphere to the gas.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of athree-stage, nitrogen, open-cycle engine according to the presentinvention;

FIG. 2 is a temperature-entropy graph for nitrogen gas showing theoperating characteristics of a threestage engine according to thepresent invention;

FIG. 3 is a schematic diagram of a three stage engine according to thepresent invention;

FIG. 4 is a fragmentary plan view of the engine mounted on an automobilerear axle;

FIG. 5 is a polar diagram of the first stage valve movement plottedagainst crankshaft position for the engine according to the presentinvention;

FIG. 6 is a fragmentary perspective view partially in section of anexhaust engine according to the present invention:

FIG. 7 is a fragmentary plan view partially in section of an alternativeexhaust engine according to the present invention;

FIG. 8 is a polar diagram of the valve movement plotted againstcrankshaft position for the valves within the exhaust engine of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A general block diagram of athree-stage open-cycle engine according to the present invention isshown in FIG. 1. Nitrogen gas in used throughout the cycle for theworking fluid of the engine. The thermodynamic states through which thenitrogen passes within the engine are diagrammatically shown byreference letters in a temperature verses entropy diagram FIG. 2. Thecorresponding locations on the block diagram FIG. 1 of thesethermodynamic states are also indicated by refer ence letters. Thenitrogen vapor engine according to the present invention operates in thesuper-heated region above the saturated liquid and saturated vapor lineson the temperature entropy diagram. Thus, the physical state of thenitrogen throughout the cycle can be characterized as mainly asuper-heated vapor at very low temperature.

Reference numeral 12 generally indicates a supply tank in which thereserve supply of liquefied nitrogen is stored. Reference letter Arepresents the state of liquid nitrogen within the supply tank, atatmospheric pressure and 77K. Connected to the supply tank 12 is a maincompressor 14. The main compressor draws the liquid nitrogen stored inthe supply tank 12 and compresses the liquid to a high pressure, stateB. The compression from state A to state B is isentropic, or at constantentropy. State B corresponds to a pressure of approximately 200atmospheres and a temperature of approximately 82K. From the maincompressor 14 the high pressure nitrogen flows into a first stage heatexchanger 16. Within the first stage heat exchanger nitrogen is heatedby a flow of ambient air to state C. The heating occurs up to atemperature of approximately 260K at a constant pressure of 200atmospheres.

The nitrogen leaves the first stage heat exchanger 16 as a gas andremains in that phase throughout the rest of the cycle.

From the first stage heat exchanger the high pressure nitrogen now atcomparatively high temperature flows into a first stage expander 18.Within the first stage expander the nitrogen undergoes an isentropicexpansion from state C to state D. Shaft work is extracted from thenitrogen while both its temperature and pressure are lowered. Thenitrogen in state D has a pressure of approximately I00 atmospheres anda temperature of approximately 195K. From the first stage expander 18,the nitrogen flows into a second stage heat exchanger 20. Within thesecond stage heat exchanger the nitrogen is again heated by a flow ofambient air to state E. This second heating occurs up to a temperatureof about 260K at a constant pressure of about atmospheres. From thesecond stage heat exchanger 20 the nitrogen next flows in to a secondstage expander 22. Within the second stage expander the nitrogenundergoes a further isentropic expansion to state F. Shaft work is againextracted from the nitrogen thus lowering both the temperature and thepressure of the nitrogen. At state F the nitrogen has a temperature ofabout K and a pressure of approximately 12 atmospheres. From the secondstage expander 22 the nitrogen flows into a recompression pump 24. Therecompression pump increases the work output of the engine cycle becausethe pump is driven by a hereinafter described process in which theenergy that would have been otherwise wasted in the exhaust of theengine is salvaged. The recompression pump raises the pressure to about20 atmospheres and raises the temperature to K, point G. The nitrogennext flows into a third stage heat exchanger 26 where the nitrogen isagain isobarically heated to state H by a flow of ambient air. At stateH the nitrogen has a pressure of about 20 atmospheres and a temperatureof approximately 260K. From the third stage heat exchanger 26 the liquidnitrogen flows into a third stage expander 28 undergoing a thirdisentropic expansion to state 1. Both the temperature and the pressureof the nitrogen are again reduced. The nitrogen leaves the third stageexpander at approximately 2 atmospheres of pressure and at a temperatureof about 135K.

From the third stage expander 28 the nitrogen flows into an exhaustengine 30. The exhaust engine salvages the last remaining energy fromthe nitrogen by a hereinafter described process. The nitrogen exhaustengine provides the work to drive the recompression pump 24 and the maincompressor 14. From the exhaust engine the nitrogen is released into theatmosphere at a temperature of about 108K.

The various components of a three-stage nitrogen vapor engine accordingto the present invention are shown in FIG. 3. The nitrogen supply tank12 is a very heavily insulated spherical tank, that is vented to theatmosphere. To reduce the evaporation of nitrogen because the tank isvented, the tank is constructed with double walls. The annular spacebetween the walls is filled with a suitable insulating material and avacuum is drawn therebetween. The tank can be constructed from eitherfiberglass or steel. The capacity of the tank determines the operatingradius of any vehicle on which the engine is mounted. For routine urbanuse, a tank having a 15 cubic foot storage capacity is adequate.

The nitrogen supply tank 12 is connected to the main compressor 14 by afuel line 32. Although the fuel line 32 is not subjected to highpressure because it is on the inlet side of the engine, the fuel line issubjected to substantial vibration from the vehicle because the storagetank 12 is remotely located from the engine. Consequently the fuel linemust be constructed from materials that are sufficiently flexible at lowtemperatures to withstand the vibration. The fuel line is well insulatedby conventional insulating materials. The insulation prevents the liquidnitrogen from vaporizing in the fuel line before the nitrogen isintroduced into the main compressor 14.

The main compressor 14 is a multiple piston, positive displacement,hydraulic pump having a variable stroke. The compressor has a variableoutput to accommodate the varying mass flow of nitrogen through theengine. To eliminate the vaporization of the nitrogen before itscompression, the compressor is heavily insulated like the supply tank 12and the fuel line 32. The pistons and the bores within the compressorare lubricated by a surface coating of solid lubricant, such as flameimpinged molybdenum. The multiple chambers within the compressor aresealed by Teflon gaskets backed by metallic springs which apply the lippressure. The compressor pump is driven by the nitrogen exhaust engine30 hereinafter described.

The compressor 14 has a stop valve 34 in fuel line 32 on the suctionside of the compressor. The stop valve 34 isolates the compressor fromthe supply tank 12. The valve 34 is shut during shut-down but is openedduring the start-up and running of the engine. The compressor 14 isconnected to the first stage heat exchanger 16 by a fuel line 38. Thefuel line 38 and all subsequent piping downstream from the compressor 14are uninsulated. After the nitrogen has been compressed, all heating ofthe nitrogen from the discharge side of the compressor 14 onward isencouraged because such heating adds energy to the nitrogen and to thecycle.

In the fuel line 38 on the discharge side of the compressor 14 is acheck valve 36. The check valve is of the ball check type and requiresno external control, working only by the differential pressure acrossits seat. The check'valve prevents the nitrogen in fuel line 38 fromflowing backward from the first stage heat exchanger 16 to the nitrogencompressor 14, thereby both preventing the heat exchanger 16 from overpressurizing the compressor during an accidental over-expansion withinthe first stage heat exchanger and also trapping the pressure within theheat exchanger during shutdown.

Connected to the discharge side of the compressor 14 by fuel line 38 isthe first stage heat exchanger 16. The first stage heat exchanger is aconventional, aircooled, tubular heat exchanger. The nitrogen passeswithin the tubes and is heated by natural convection. Natural convectionis sufficient because when the vehicle is stationary the engine does notconsume fuel and thus does not require any heat transfer. When thevehicle is moving and consuming fuel, the heat exchanger transferssufficient heat from the flow of ambient air around the exposed tubes,which are located in the path of the air stream passing by the vehicle.The heat exchanger tubes can be constructed from any suitable metallicmaterial, such as steel or stainless steel.

The ice which forms on the tubes of the heat exchanger from thecondensation of water vapor in the air is removed by a system of brushesthat are cyclically passed through the tube bank. The brushes aremounted on a moving sub-frame that passes between the tubes. The axes ofthe brushes are parallel to the axes of the tubes. The tubes arecylindrical in cross section and do not have fins which would obstructthe passage of the moving sub-frame. The brushes are cylindrical andhave metallic bristles to multiply the heat transfer from the air to thetubes.

The first stage heat exchanger has a relief valve 40. The relief valveis a conventional high pressure relief valve and is located on one ofthe tubes carrying nitrogen. Since the first stage heat exchanger 16 issubjected to the highest pressure in the engine, the relief valve 40provides pressure protection for the entire system.

The first stage heat exchanger 16 is connected to the first stageexpander 18 by a fuel line 42. The fuel line is uninsulated because anyheat transfer from the air to the nitrogen within the fuel line addsenergy to the cycle. On the fuel line is located a stop valve 44. Thestop valve isolates the first stage heat exchanger 16 from the firststage expander 18. The stop valve is shut during shut-down to maintainpressure within the heat exchanger but is open during the start-up andrunning of the engine.

The first stage expander 18 is comprised of a head 56 and two axiallyconcentric cylinders, an upper cylinder 48 and a lower cylinder 50. Thehead and cylinders are held together by long steel studs, not shown,which threadably connect to a common block 90. The diameter of the uppercylinder is substantially smaller than the lower cylinder. Within bothcylinders travels a unitary piston 52. The piston is cylindrical inshape and has an upper section having a substantially smaller diameterthan the lower section. The expansion of the nitrogen occurs within theupper cylinder 48 against the upper, smaller diameter section of thepiston. The lower section of the piston having the larger diameter takesup the thrust from the upper cylinder 48. The piston and cylinders canbe manufactured from any suitable metallic material such as steel oraluminum. The piston is sealed by a packing 54. The packing is Teflonbacked up by a steel spring.

Within the head 56 of the first stage expander 18 is a valve block 46having an inlet valve and an exhaust valve, not shown. Both valves arepressure balanced solenoid operated, spool valves of the type typicallyused in hydraulic systems. The operation of the solenoids is controlledby conventional 12 volt D.C. circuits. The inlet and exhaust valves arethe same size because although the nitrogen has expanded within thecylinder during the cycle, the exhaust valve remains open for asubstantially longer period of time than the inlet valve.

The first stage expander 18 is connected to the second stage heatexchanger 20 by a fuel line 58. The fuel line 58 is uninsulated in orderto promote further heat transfer from the surrounding air. The fuel linecontains along its length a check valve 60. The check valve is of theball check type requiring no external control. The check valve preventsthe backflow of the nitrogen in the fuel line 58 from the second stageheat exchanger 20 to the first stage expander 18. By preventing thisbackflow, the check valve maintains the nitrogen pressure within theheat exchanger during shut-down and prevents the heat exchanger fromover-pressurizing the expander because of an accidental thermaltransient.

The fuel line 58 is directly connected to the second stage heatexchanger 20. The second stage heat exchanger is a conventional,air-cooled, tubular heat exchanger that is unfinned. The second stageheat exchanger is constructed like and operates similarly to the firststage heat exchanger 16. All three heat exchangers 16, 120 and 26 areconstructed as one integral unit with a common ice removing system asdescribed hereinbefore. Within the second stage heat exchanger 20 thenitrogen travels within the tubes and is heated by natural convection.The heat transfer area of the second stage heat exchanger is,substantially smaller than the area of the first stage heat exchangerbecause the engine requires less heat transfer at this juncture in thecycle. Quantitatively, the area under the graph of the cycle in FIG. 2which represents heat in the cycle is less under the line segment DEthan under the line segment BC.

The second stage heat exchanger 20 and the second stage expander 22 areconnected by a fuel line 62. The fuel line 62 is uninsulated to promotefurther heat transfer from the surrounding air. The fuel line 62contains a stop valve 64. The stop valve isolates the second stage heatexchanger 20 from the second stage expander 22. The stop valve is shutduring shut-down to maintain the nitrogen pressure within the heatexchanger but is opened during the start-up and running of the engine.

The second stage expander 22 is comprised ofa head 212 and a singlecylinder 214. The head and the cylinder threadably engage each other andare retained in place by long steel studs, not shown, which threadablyconnect to a common block 90. Within the cylinder 214 travels a piston102. The piston is sealed by a packing 216. The packing is a Teflon ringbacked up by a steel spring. The nitrogen expansion occurs within thechamber 218 formed by the cylinder walls, the head, and the top surfaceof the piston. Within the head 212 is a valve block having an inletvalve and an exhaust valve, not shown. Both valves are pressurebalanced, solenoid operated, spool valves of the type typically used inhydraulic systems. These valves are constructed and operate in a similarmanner as the valves within the head 56 of the first stage expanderhereinbefore described.

The second stage expander is connected to the recompression pump 24 by afuel line 66 which is uninsulated. The recompression pump 24 is amultiple piston positive displacement, hydraulic pump with a variablestroke to control the output. The recompression pump has a variableoutput in order to accommodate the varying mass flow rates of nitrogenat differing engine speeds and power settings. The pistons and bores(not shown) within the recompression pump are lubricated by surfacecoatings of solid lubricants, such as flame impinged molybdenum. Thecylinders and pistons within the recompression pump are sealed by Teflonseals backed by metallic springs. The recompression pump is driven bythe exhaust engine 30 hereinafter described. The recompression pump isprimarily used in this thermodynamic cycle to increase the work outputof the engine and consequently the overall efficiancy. The increasedoutput when using a recompression pump results because the recompressionpump is driven by the energy that would otherwise have been lost to theatmosphere in the exhaust.

The recompression pump 24 is connected to the third stage heat exchanger26 by a fuel line 68, which is uninsulated. In the fuel line on thedischarge side of the recompression pump 24 is a check valve 70. Thecheck valve is a ball check valve similar to check valve 36 and islocated to prevent the backflow of the nitrogen in fuel line 68 from thethird stage heat exchanger 26 to the recompression pump 24. Bypreventing this backflow, the check valve maintains the nitrogenpressure within the third stage heat exchanger 26 during shut-down andprevents the third stage heat exchanger from over-pressurizing therecompression pump 24 because of an accidental thermal transient.

The third stage heat exchanger 26 receives the nitrogen discharged bythe recompression pump 24 through the fuel line 68. The third stage heatexchanger is a conventional air-cooled, tubular heat exchanger that isunfinned. Within the third stage heat exchanger the nitrogen travelswithin the tubes and is heated by natural convection. The third stageheat exchanger is constructed like and operates similarly to the firstand second stage heat exchangers 16 and 20 described hereinbefore. Allthree heat exchangers are constructed as one integral unit with a commonice removing system.

The third stage heat exchanger 26 and the third stage expander areconnected by a fuel line 72. The fuel line is uninsulated to promotefurther heat transfer. The fuel line contains a stop valve 74. The stopvalve 74 isolates the third stage expander from the third stage heatexchanger. The stop valve is shut during shutdown to maintain thenitrogen pressure within the third stage heat exchanger but is openedduring the start-up and running of the engine.

The third stage expander 28 is comprised of a head 80 and a singlecylinder 78. The head and the cylinder threadably engage each other andare retained in place by long steel studs, not shown, which threadablyconnect to a common block 90. Within the cylinder 78 travels a piston76. The piston is sealed by a packing 82. The packing is a Teflon ringbacked up by a steel spring. The construction and operation of the thirdstage expander is similar to the second stage expander 22 hereinbeforedescribed. The nitrogen expansion occurs within the chamber 83 formed bythe cylinder walls, the head, and the top surface of the piston. Thechamber 83 is larger than the second stage chamber 218 which is, inturn, larger than the first stage chamber 48. This increase in chambersize accommodates the increase in specific volume of the nitrogenbetween the stages. Within the head 80 is a valve block having an inletvalve and an exhaust valve, not shown. Both valves are pressurebalanced, solenoid operated, spool valves of the type typically used inhydraulic systems. These valves are constructed and operate in a similarmanner as the first and second stage valves hereinbefore described.

The third stage expander is connected to the nitrogen exhaust engine 30by a fuel line 84. The fuel line 84 is uninsulated to promote furtherheat transfer. The fuel line contains a check valve 86. The check valve86 is a ball check valve similar to check valve 60 and is located toprevent backflow from the exhaust engine 30 to the third stage expander28. By preventing this backflow, the check valve prevents the exhaustengine from over-pressurizing the third stage expander. From the exhaustheat engine 30, the nitrogen leaves the engine through fuel line 88 andis exhausted to the atmosphere.

Although only three expanders and three heat exchangers are shown anddescribed, it should be obvious that additional or fewer expanders andheat exchangers can be combined. Likewise, the number of pumps may beincreased or decreased and the construction, operation and sequence ofthe valves can be varied without departing from the scope of theinvention.

The three expanders 18, 22 and 28 are housed on a common block 90. Thethree cylinders are disposed in a radial configuration with a 120separation between the axis of each cylinder. Within the block 90 is asingle, common, longitudinal crank shaft 92. The crank shaft 92 has asingle throw 94 because of the three cylinders. The main bearings forthe crank shaft are tapered roller bearings, not shown. Attached to thecrank shaft 92 are three connecting rods 96, 98 and 100. The connectingrods mechanically couple the pistons 52, 102 and 76 to the crank shaft92 by wrist pins 104, 106 and 108, respectively. The bearings 110 forthe three connecting rods on the crank shaft are double sealed, needlebearings with inner and outer races. The bearings 112, 114 and 116 forthe connecting rods at wrist pins are bronze or graphite-filled Teflonbearings. Internal oil lubrication is not required because the engine isdesigned for a maximum speed of 800 rpm. Hence, the main bearings andthe rod bearings 110 are lubricated by conventional grease fittings, notshown.

The engine is mounted at the rear of a small automobile. Referringspecifically to FIG. 4, the block 90 of the engine is disposed so thatthe crank shaft 92 is parallel and elevated above the rear axle 117 ofthe automobile. The drive shaft and universal joints to the rear axlehave been removed. At the remote end of the crank shaft directly overthe differential gear box 119 is a sprocket 118. The sprocket is keyedto the crank shaft in order to withstand the high torque required forstarting the automobile. Attached to the sprocket 118 is a roller chain120. The roller chain can be either a multiple strand roller chain or aninverted tooth, silent chain. The chain 120 connects directly to asprocket mounted on the ring gear carrier, not shown, within thedifferential gear box 119 in place of the conventional bevelled ringgear or crown wheel. The sprocket 118 is the same size as the sprocketwithin the differential gear box so the engine has a one-to-one drive tothe rear axle. If a change in gear ratio is desired, sprocket 118 can beeasily changed. This power train configuration eliminates the drivingpinion within the differential, the drive shaft, the universal joints,the transmission and the clutch which are all required on conventionalgasoline powered automobiles.

At the other, lateral end of the engine, remote from the sprocket 118,is the valve timing assembly 122. Within the valve timing assembly 122are slip rings, not shown, that are attached to the crank shaft 92 toprovide the electrical timing contact for the valves. This timingcontact is used to open and shut in proper sequence the inlet andexhaust valves within each valve block on the expanders. The timingsequence of the valves with respect to the crankshaft position is variedby using slow twist, multiple threads in the slip rings that permit theslip rings to be rotated infinitesimally around the crank shaft. Thevalve timing assembly 122 is connected to the solenoids on therespective valve blocks by the electrical cables 123.

Referring to FIG. 5, the valve timing for the first stage cylindershowing the cycling of the inlet and exhaust valves is plotted againstcrank shaft position on a polar graph. When the crank shaft is at topdead center (T.D.C.) or the zero degree mark in FIG. 5, the piston 52 isat the top of its stroke and both the inlet and exhaust valves are shut.As the crank shaft travels a small angle past zero degrees top deadcenter and the piston commences its downward stroke, the inlet valveopens. The inlet valve always opens at the same crank shaft angleregardless of the throttle setting or engine speed. However, the inletvalve remains open for a variable interval. In this embodiment of thepresent invention, the point at which the inlet valve closes iscontrolled by the throttle setting. The speed of the engine and thetorque it develops are directly proportional to the volume of nitrogenintroduced to each cylinder. By varying inlet valve closing time, thesize of the charge of nitrogen introduced into the cylinder is regulatedand thus the engine is able to vary in speed. For a greater throttleopening, the inlet valve closes later. At a maximum throttle setting,the inlet valve closes no later than 20 past TDC. After the inlet valveshuts, the charge of nitrogen within the cylinder expands and the pistonis forced downward. When the piston is at the bottom of the stroke, thebottom dead center point (BDC) or 180 of crank shaft rotation, theexhaust valve opens. The exhaust valve remains open through the next 180of crank shaft rotation as the piston returns upward. The exhaust valveshuts just before TDC and just before the piston reaches the top ofstroke. The exhaust valve opens and shuts invariably at the same pointsduring each rotation of the crank shaft. The inlet and exhaust valvesare so sequenced that at TDC neither valve is open. Just prior to TDCthe exhaust valve shuts and just after TDC the inlet valve opens. Thistiming is required so that at no time are both valves simultaneouslyopen. In that case, the incoming nitrogen would blow out the exhaustline without undergoing expansion in the cylinder.

The operation and sequencing of the valves in the second and third stageexpanders are analogous to the first stage expander and differ only in aphase angle of of crank shaft rotation. Whereas the first stage inletvalve opens soon after TDC, the second stage inlet valve opens soonafter TDC 120 and the third stage inlet valve opens soon after TDC 120.The other valves open and close similarly.

The electrical solenoid control of the sequencing of the valves permitsthe engine to start either in a forward or reverse direction. In fact,the engine can develop maximum speed in either direction. Moreover, anyinaccuracy in the sequencing between the crank shaft and the motion ofthe valves merely results in the throttle being in a slightly differentposition than the throttle ordinarily would be for the same torqueoutput. This timing problem is common to internal combustion gasolineengines and is primarily caused by the speed of the valves beingconstant while the speed of the engine varies. However, while impropertiming causes improper operation of an internal combustion engine,improper timing on a vapor engine only causes a change in throttleposition.

The nitrogen vapor engine has a priming system that is used to charge upthe system with nitrogen prior to operation. Basically, the primingsystem fills each one of the heat exchangers with nitrogen. Once theheat exchangers are filled, the nitrogen heats up from the ambient airand system pressure builds up rapidly. The nitrogen priming systemconsists of a small hand pump 124. This hand pump takes a suction onfuel line 32 at the outlet of the nitrogen storage tank 12 and the handpump discharges through fuel lines 126, 128 and 130. These fuel linesare connected to the heat exchangers 16, and 26 in each stage. On eachone of these fuel lines 126, 128 and 130 is a solenoid operated stopvalve respectively 132, 134 and 136 which is used to control the flow ofnitrogen during priming. These valves ae normally shut and only openedwhen the system is being primed.

The start-up of the nitrogen vapor engine is initiated by filling thenitrogen supply tank 12 with liquid nitrogen. Stop valve 34 is shut andthe priming pump stop valves 132, 134 and 136 are opened. Next thepriming pump 124 which takes a suction directly on the fuel line 32leading from the supply tank 12 fills the heat exchangers 16, 20 and 26with nitrogen. The heat exchanger outlet stop valves 44, 64 and 74 areshut to allow the priming pump to build up pressure within the heatexchangers. Once the heat exchangers have been primed, the priming pumpvalves 132, 134 and 136 are shut. The nitrogen gas pressure within theheat exchangers builds up rapidly because the heat exchangers were atambient temperature before filling. When the pressure is sufficientlyhigh, the heat exchanger outlet valves 44, 64 and 74 are opened, thuscharging the fuel lines to the inlet sides of each one of the cylinders.At this point, the engine is ready to operate although at reduced power.

To operate the engine after being filled with nitrogen, the throttlemotion causes one of the inlet valves to one of the cylinders to open.The initial position of the crank shaft determines which inlet valveopens. As soon as this first nitrogen charge is introduced into acylinder that charge causes the piston within that cylinder to rotatethe crank shaft. Before the crank shaft has turned 120, another inletvalve has opened. The second cylinder then begins to contribute to thecrank shaft rotation. Once the crank shaft is turning, the normal timingcircuits sequence the valves. After another 120 of crank shaft rotationthe third inlet valve opens, and after a complete revolution all valveshave cycled. The throttle motion also starts the main compressor 14 thatquickly builds up the nitrogen pressure throughout the engine. As soonas the pressure is up to normal, the engine is capable of operating atmaximum power.

The priming pump is required only when the system has been opened to theatmosphere and warm, humid air has been admitted into the engine. If theengine has been operating on nitrogen, and if the nitrogen pressure canbe maintained within the heat exchangers during shutdown, then thepriming pump is not needed. In this case merely opening the throttle isall that is necessary for starting up. The motion of the throttle willopen an inlet valve and the crank shaft will start to turn.

To shut down the nitrogen engine, the heat exchanger exhaust valves 44,64 and 74 are shut electrically. When these valves shut, the charge ofnitrogen within each heat exchanger is retained therein. The reversebackflow out of the heat exchangers is prevented by the ball checkvalves 36, 60 and in the inlet fuel lines to each heat exchanger.

The operating characteristics of the nitrogen engine are very similar tothose of a steam locomotive. The engine develops maximum torque at zerospeed and has very flat torque response as the engine speed increases.The flat torque response terminates when a speed is reached where thenitrogen flow losses become significant. Horse power is a linearfunction of speed in the region of constant torque response. The engineoperates up to a maximum speed of approximately 800 rpm. The consumptionof fuel increases as speed increases if constant torque is maintained.The efficiency of the engine is a direct function of the size of thecharge of nitrogen admitted into each cylinder. This charge size iscontrolled by the throttle opening. When the optimum size charge isadmitted into the cylinders, the engine will accelerate until therolling resistance, the air drag and the frictional resistance of thewhole drive line balance the torque available.

In order to extract the last remaining energy from the nitrogen beforeit is exhausted to the atmosphere, the nitrogen engine utilizes anexhaust engine 30. More specifically, one embodiment of the nitrogenexhaust engine is shown in FIG. 6. Reference numeral 138 generallyindicates a long, rectangular box that is well insulated in the inside.The box has a remote end wall 144 and a central end wall, not shown,which oppose each other. Near the central end wall within the box is asliding plate or piston 146. The sliding plate has four longitudinalrunners 148, 150, 152 and 154. These runners fit into longitudinalguides 156 that are located on the interior surfaces of the upper andlower sidewalls of the box. The longitudinal runners in combination withthe longitudinal guides permit the sliding plate 146 to slidereciprocally along the longitudinal axis of the box while remainingperpendicular to the sides of the box. The sliding plate can freelytraverse the interior of the box from one end wall to the other end wallwithout tipping or wedging between the sidewalls.

Rigidly attached to the interior facing side of the remote end wall 144and the interior facing side of the sliding plate 146 are numerouspulleys 142. Strung between these pulleys are a plurality of wires 140.These wires are strong, of small diameter, and highly stressed. Thewires are rigidly attached to either the end wall 144 or the slidingplate 146. Each wire is strung over at least one pulley so that eachwire makes at least two transits of the length of the box. To minimizethe number of attachment points for the wire ends because the wires areunder substantial tension, it is preferable to have each wire makenumerous transits of the box.

On the exterior facing side of the sliding plate 146, the side obverseto which the pulleys are mounted, is a pin and connecting rod 158. Theconnecting rod is attached to a crank shaft 159. The crank shaft 159 canbe either directly connected to the main compressor 14 or therecompression pump 24 or can be connected to an electrical generator ofthe conventional type. The linear motion of one sliding plate isdirectly and reciprocally counterbalanced by another sliding platelocated directly opposite in a corresponding box. Both sliding platesare connected to the common crank shaft 159 by identical linkage. Eachbox is of similar construction and is disposed with respect to eachother so that one box pulls against the other box in order to keep allthe wires within both boxes tight. A substantial tension of the wirescan thus be maintained.

Reference numeral 160 generally indicates an intake manifold forinducting the nitrogen vapor into the exhaust engine 138. The intakemanifold consists of a nitrogen fuel line 84 coming directly from thethird stage expander 80. Accompanying the nitrogen inlet 84 is an airinlet 164. The air inlet leads directly from the exterior of theautomobile where the air inlet is pointed into the oncoming airstream.Both the nitrogen fuel line 84 and the air inlet 164 are connected to avalve block 166. Within the valve block is a solenoid operated, convtrolvalve, not shown, which selectively and individually ducts either air ornitrogen into the intake manifold 160. The intake manifold terminates atnumerous intake orifices in the sidewall of the box 138. These orificeslead directly into the interior of the box.

On the other side of the box 138 opposite from the intake manifold 160is an exhaust manifold 168. The exhaust manifold is connected tonumerous exhaust orifices on the opposing sidewall from the intakeorifices. The exhaust manifold terminates at a valve block 172. Withinthe valve block is another solenoid operated, control valve, not shown,that selectively and individually directs the exhaust from the boxeither into a nitrogen exhaust pipe 88 or an air exhaust pipe 170. Bothof these pipes ultimately discharge into the atmosphere.

Although only two boxes are shown and described, it should be obviousthat additional boxes and manifolds can be combined to achieve greaterengine efficiency. Further, one box can be used alone if some spring ortensioning mechanism is used to maintain the tension on the wires. Adouble acting piston or a series of plates which reciprocally drive eachother can be more efficient although they are not required to practicethis invention.

The exhaust engine operates by sequentially altemating the fluid flowthrough each box 138 between cold nitrogen and relatively warmer,ambient air. The cold nitrogen causes the wires within the box tocontract and the warm air causes the wires to expand. The expansion andcontraction of the wires translates into linear motion of the slidingplate 146. Because the wires have small cross section, are kept underhigh stress, and have a long length, the movement of the sliding plateis substantial. Two boxes can operate together by introducing into onethe warmer air while introducing into the other the colder nitrogen.Thus, while in one box the wires are contracting, in the other box thewires are expanding. The inlet and exhaust valves to each box are sotimed that the two sliding plates reciprocally oscillate andcounterbalance each other. The sliding plates are mechanically connectedaround a common crank shaft 159 so the linear motion of the slidingplates is translated into rotational motion of the crank shaft.

An alternative embodiment of the exhaust engine according to the presentinvention is shown in FIG. 7. This embodiment utilizes a plurality ofsmall heat exchangers and a double acting piston. More specifically,reference numeral 174 generally indicates an intake manifold. The intakemanifold consists of several identical arms or heat exchanger tubes 176mounted on a common header 177. Located at the end of each heatexchanger tube 176 remote from the header is an inlet valve 178. Locatedat the central end of each heat exchanger tube 176 near the header is anexhaust valve 180. Both the inlet and outlet valves are solenoidactuated stop valves which can completely isolate each tube 176 from theheader 177 and from the fuel line 84. Nitrogen from the third stageexpander is induced into the intake manifold 174 through the fuel line84. The nitrogen is selectively introduced into one of the heat transfertubes 176 by sequentially cycling the inlet valves 178.

The header 177 leads directly to the double acting piston assembly 182.The piston assembly has an upper inlet valve 184 and a lower inlet valve186. These inlet valves are solenoid actuated stop valves that controlthe entrance of nitrogen into the piston assembly. In addition, thepiston assembly has an upper exhaust valve 188 and a lower exhaust valve190. These valves are also solenoid actuated stop valves of theconventional type. These exhaust valves control the exit of nitrogenfrom the piston assembly through exhaust line 88. Within the pistonassembly, reference numeral 192 indicates a movable piston whichreciprocally oscillates between the upper and lower chambers of thepiston assembly. The piston 192 is sealed by a Teflon ring 202, backedup by a metallic ring, not shown. The Teflon ring 202 primarily preventsthe leakage of nitrogen pressure between the upper and lower cylindersand provides a bearing surface for the piston 192 as it oscillateswithin the cylinder. The piston has a wrist pin 194 which attaches theconnecting rod 196 to the piston. The connecting rod further attaches toanother wrist pin 198 mechanically connected to an upper piston 200. Theupper piston 200 guides the double acting piston 192 and absorbs the rodthrust during the reciprocal motion of the piston 192. The upper piston200 is connected to a crank shaft 204 with conventional mechanicallinkage. The crank shaft 204 can be either directly connected to themain compressor 14 or to the recompression pump 24 or can be connectedto an electrical generator of the conventional type.

Although only one intake manifold and one double acting piston assemblyare shown and described, it should be obvious that additional manifoldsand additional piston assemblies can be combined to achieve greaterengine efficiency. Moreover, the design, operation, and sequence of thevalves can be varied without departing from the scope of this invention.

In operation, moderately cold nitrogen exhausted from the main engine atapproximately atmospheric pressure enters the exhaust engine through thefuel line 84. The nitrogen flows into one of the duplicate heatexchanger tubes 176 through an open inlet valve 178. The complementaryexhaust valve is controlled by a temperature sensor, not shown, sensingthe heat exchanger tube temperature. When the temperature sensorcontrolling the exhaust valve registers that the cold nitrogen isentering the tube, the temperature sensor shuts the exhaust valve. Theinlet valve is controlled by a pressure sensor, not shown, that sensesthe pressure within the tube between the inlet and exhaust valves. Whenthe pressure builds up sufficiently within the heat exchanger tube, thepressure sensor controlling the inlet valve shuts it. Thus, both inletvalve 178 and exhaust valve 180 are shut entrapping a charge of coldnitrogen in the tube therebetween. In the meanwhile, atmospheric air atambient temperature is either continuously ducted or naturallycirculated by the heat exchanger tubes. This atmospheric airconvectively heats the charge of nitrogen within each tube and raisesits pressure to a maximum, approximately three atmospheres. At theappropriate time, depending on the timing of the crank shaft, thepressure within the tube and the relative pressure among the othertubes, the exhaust valve 180 opens to charge the header 177 and to forcethe piston 192 to the opposite end of the cylinder. When the piston hasmoved within the chamber to the end of its stroke, the correspondingexhaust valve opens to exhaust the nitrogen from that chamber and toallow the return travel of the piston. The process then repeats itselfas the header 177 discharges into the opposite chamber and nitrogenforces the piston into a return stroke.

The time required for each tube to heat sufficiently in order togenerate the necessary pressure is relatively long compared with theother processes in the engine. Hence, there are numerous heat exchangertubes accompanying each piston assembly so each tube has ample time toreach its maximum pressure before being opened onto the header.

Referring to FIG. 8, the sequence of valve operation for the doubleacting piston assembly 182 is shown plotted against degrees of crankshaft rotation on a polar diagram. At top dead center (TDC) of crankshaft rotation or the piston 192 is at the top of its stroke and theupper chamber volume of the piston assembly is at a minimum. At bottomdead center (BDC) of crank shaft rotation or 180, the piston is at thebottom of its stroke and the upper chamber volume of the piston assemblyis at a maximum. By the design of the piston assembly when the upperchamber volume is at a minimum, the lower chamber volume is at a maximumand vice versa.

At TDC the following valves are already open: the inlet valve 178 andthe exhaust valve 180' to one heat exchanger tube 176' and the upperinlet valve 184 and the upper exhaust valve 188 to the upper chamber.With these valves open there is a direct open path between inlet fuelline 84 and the exhaust line 88. This open path allows nitrogen from theinlet fuel line 84 to purge the open heat exchanger tube, the header,and the upper chamber.

Soon after TDC the upper exhaust valve 188 shuts. The heat exchangerexhaust valve 180' also shuts from the temperature drop from the coldnitrogen entering the tube 176. Next. the exhaust valve 180 for adifferent heat exchanger tube opens pressurizing the header.Simultaneously, the lower exhaust valve 190 for the lower chamber alsoopens. With valves 180, 184, and 190 open and valves 178, 186, and 188shut, the piston begins its downward power stroke under the force of thenitrogen from tube 176.

Before BDC the upper inlet valve 184 shuts and both the inlet valve 178to the currently operating heat exchanger tube 176 and the lower inletvalve 186 to the lower chamber open. These valves open a new purge paththrough the operating heat exchanger tube, the header, and the lowerchamber.

Soon after BDC the lower exhaust valve 190 to the lower chamber shutsalong with the heat exchanger exhaust valve 180. Next, the exhaust valvefor a different heat exchanger tube opens pressurizing the headersimultaneously with the opening of upper exhaust valve 188 of the upperchamber. With valves 180, 186, and 188 open and valves 178', 180, 184,and 190 shut, the piston begins its upward power stroke under the forceof the nitrogen from tube 176'.

Before TDC the lower inlet valve 186 to the lower chamber shuts and boththe inlet valve 178' to the currently operating heat exchanger tube 176and the upper inlet valve 184 to the upper chamber open, returning tothe initial condition.

Although only two heat exchanger tubes are described in the sequencediagrammed in FIG. 8, it is intended that all of the tubes are to beused sequentially as the pressure in each reaches a maximum value.

Although several embodiments of the present invention have been shownand described, it will be obvious that other adaptations andmodifications can be made to this invention without departing from thetrue spirit and scope of the invention.

What is claimed is:

1. An improved thermodynamic method of producing mechanical energy froma fluid, said method comprising the steps of:

a. insulatively storing said fluid at a temperature substantially belowambient;

b. isothermally pumping said fluid to a pressure above atmosphericpressure;

c. performing at least twice the sequential steps of:

i. isobarically heating said fluid by passing said fluid through a heatexchanger having an exterior surface in thermal contact with ambient;and ii. isentropically expanding said fluid in an expansion engine toproduce mechanical energy; and cl. isentropically compressing said fluidafter one of said steps of isentropically expanding said fluid andbefore the succeeding step of isobarically heating said fluid.

2. The method of claim 1 further including the step of removingaccumulated ice from said exterior surface of at least one of said heatexchangers to maintain the thermal transfer efficiency thereof.

3. The method of claim 1 further including the steps of:

e. converting to mechanical energy the thermal energy remaining in saidfluid after the last of said steps of isentropically expanding saidfluid; and

f. using the mechanical energy obtained by step (e) to perform step (d)of isentropically compressing said fluid.

4. The method of claim 3 further including the step of using a portionof the mechanical energy obtained by step (e) to perform step (b) ofisothermally pumping said fluid.

1. An improved thermodynamic method of producing mechanical energy froma fluid, said method comprising the steps of: a. insulatively storingsaid fluid at a temperature substantially below ambient; b. isothermallypumping said fluid to a pressure above atmospheric pressure; c.performing at least twice the sequential steps of: i. isobaricallyheating said fluid by passing said fluid through a heat exchanger havingan exterior surface in thermal contact with ambient; and ii.isentropically expanding said fluid in an expansion engine to producemechanical energy; and d. isentropically compressing said fluid afterone of said steps of isentropically expanding said fluid and before thesucceeding step of isobarically heating said fluid.
 2. The method ofclaim 1 further including the step of removing accumulated ice from saidexterior surface of at least one of said heat exchangers to maintain thethermal transfer efficiency thereof.
 3. The method of claim 1 furtherincluding the steps of: e. converting to mechanical energy the thermalenergy remaining in said fluid after the last of said steps ofisentropically expanding said fluid; and f. using the mechanical energyobtained by step (e) to perform step (d) of isentropically compressingsaid fluid.
 4. The method of claim 3 further including the step of usinga portion of the mechanical energy obtained by step (e) to perform step(b) of isothermally pumping said fluid.